Abstract

Telomeres are specialised structures that protect chromosome ends from degradation and fusion events. In most eukaryotes,
including yeast and mammals, telomeric DNA consists of short, repetitive G‐rich sequences, which end with 3′ G‐rich overhangs
and are elongated by the specialised reverse transcriptase telomerase. Telomeric repeats are bound by several DNA‐binding
proteins, which regulate telomerase activity and protect (cap) chromosome ends from degradation and inappropriate DNA repair.
If uncapped, telomeres are sensed as DNA breaks and undergo unwanted DNA repair, which can eventually lead to the activation
of cell cycle checkpoints and/or to end‐to‐end fusion. The dicentric chromosomes generated by telomeric fusion can cause non‐disjunction
and chromosome breakage during anaphase. These events result in loss of genetic material and chromosome rearrangements that
in mammals might lead to several diseases including cancer. Thus, it has become evident that telomeres play critical roles
in the maintenance of genome stability.

Key Concepts

Telomeres are required for chromosome end maintenance and genome stability.

Telomeres are special chromatin architectures found at the end of linear chromosomes

Telomeres protects chromosome ends from degradation and fusion events

Telomeric DNA consists of tandem repeats that are bound by several DNA-binding proteins

Critically short telomeres become dysfunctional and eventually lead to chromosome fusions and cell death

Figure 1.Telomere capping complexes in different species. In mammals, the six‐member shelterin complex associates with both single‐ and double‐stranded regions of the telomeric DNA. S. cerevisiae telomeres are protected by the trimeric Cdc13/Stn1/Ten1 (CST) complex, which assembles on the G‐overhang. The Rap1/Rif1/Rif2 complex binds the duplex region of the budding yeast telomere. S. pombe telomeres are bound by a shelterin‐like complex. In addition, Stn1 and Ten1 contribute to chromosome end protection, but it is not known how they interact with other telomere proteins. D. melanogaster Terminin contains the fast evolving proteins HipHop, HOAP, Moi and Ver. Ver is structurally related to human Stn1 and binds ss‐DNA. In A. Thaliana, CST functions as the major telomere capping complex. POT1a is a telomerase accessory factor and is not required for chromosome end protection.